1. Field of the Invention
The present invention relates to mechanical seals, and, more specifically, to mechanical seals mounted in compressors, e.g. swash-plate type compressors used in air-conditioners for automobiles.
2. Description of the Prior Art
Mechanical seals are widely used to prevent the flow of liquids or gases in moving parts of machine elements (see, Packing Committee: MECHANICAL SEALS AND PACKINGS, Tool Engineers Handbook (Second Edition), McGraw-Hill, 1959, 97-1-97-7). For example, a mechanical seal is mounted in a swash-plate type compressor disclosed in U.S. Pat. No. 3,999,893.
Generally, a mechanical seal comprises (1) a sealing ring set onto a rotating shaft and rotating, (2) a stationary mating ring which has a fine-finished surface coming into contact with the sealing ring and (3) means for pressing the sealing ring against the mating ring. The rotating surface of the sealing ring comes into contact with the stationary surface of the mating ring at right angles to the shaft to form a sealing face. The sealing ring is, usually, of sintered carbon-graphite, resin-bonded carbon-graphite, resin-impregnated carbon-graphite or ceramic and the mating ring is of cast iron, stainless steel, stellite-faced stainless steel, ceramic or hard chromium-plated steel.
In a case where a mechanical seal is mounted in a swash-plate type compressor of an air-conditioner for an automobile, the operational conditions of the mechanical seal become considerably more severe. Namely, since the drive source of the compressor is an internal combustion engine, i.e., a gasoline engine or a diesel engine, the sealing ring of the mechanical seal fitted onto the drive shaft of the compressor is rotated at almost the same rotational speed as that of the internal combustion engine. Accordingly, the sealing ring is subjected to a rotational rate of 500 rpm when the internal combustion engine is idling, and to a rotational rate of 6000 rpm during high speed travel or rapid acceleration, so that variation of the operating speed of the mechanical seal is very large. Furthermore, since the sliding movement of the sealing ring with respect to the stationary mating ring can be performed by pressing the sealing ring against the mating ring by a spring, the sliding surfaces are constantly maintained under a boundary lubrication, namely, parts of the sliding surfaces are brought into contact with each other in a solid contact. Although the mechanical seal seals a sealing fluid of a mixture of a refrigerant gas and mist of lubricating oil within the compressor, a small amount of the lubricating oil leaks and thus, the lubrication between the sliding surfaces is unsatisfactory. However, during a period of several tens of seconds or sometimes a period of a few minutes, after the starting of the compressor, the sealing ring slides on the mating ring without the lubricating oil.
In a case where cast iron is used as a material for the stationary mating ring of the above-mentioned mechanical seal, though the surface of the ring is fine finished to decrease the surface roughness, the sealing capacity of the mechanical seal is low. In this case, graphites distributed within the cast iron are taken off to form small hollows by fine finishing. Furthermore, the hardness of the cast iron is relatively low, such as up to H.sub.V (Vickers hardness) 300, so that the wear rate of the cast iron is relatively high. As the wear increases, the surface roughness of the mating ring becomes uneven. The small hollows and the uneven roughness result in a decrease of the sealing capacity.
In a case where a ceramic is used as a material of the stationary mating ring, the mechanical seal has several disadvantages. Namely, the ceramic ring may be cracked by rapid variation of temperature (e.g. thermal impulse) generated by rapid acceleration to a high speed. High hardness and high wear-resistance of the sliding surface of the mating ring are advisable for the sealing capacity of the mechanical seal. However, it is difficult to machine the ceramic to a predetermined ring shape. Furthermore, since the ceramic material has a low heat conductivity and a high coefficient of friction, the sliding surfaces of the ceramic mating ring and the sealing ring generate heat and thus the sealing face reaches a high temperature. As a result, leakage of liquids or gases from the mechanical seal increases.
It is undesirable to use stainless steel as a material of the stationary ring of the above-mentioned mechanical seal. Since the stainless steel has a low heat conductivity, the sliding portion of the stainless mating ring is heated by frictional heat, as a result, the strain generated by thermal stress decreases the sealing capacity.
Hard chromium-plated steel has desirable properties as a material for the stationary mating ring except that the coefficient of friction between the mating ring and the carbon-graphite sealing ring increases when the mechanical seal is operated under severe conditions, i.e. at a high load without lubricating oil.